ECLSS was a department in Starfleet Command's Fleet Operations Center on Earth.
In January of 2155, Starfleet officers B. Blackman, M. Moore, J. Pizston, and J. Wischnack all worked in ECLSS. (ENT: "Demons", "Terra Prime")
ECLSS, in the real world, stands for Environmental Control and Life Support System. See life support system at Wikipedia for more information.